这次排错过程主要是思路，视野打开后会觉得豁然开朗，原来这其实是个小问题[尴尬]。
1、没注重应用启动的各服务及其用途，只会简单查看 status；
2、看到错误第一时间想到的是 Baidu（没其他意思），找找 logpath 先看日志不好吗？
3、未认识到服务之间的关联关系（比如 postgresql 与 unicorn 之间），前面一直知道 unicorn 启动后没正常监听到端口，但是日志并没啥特别信息（嗯，可能是因为看错了文件）[苦笑]
一、错误信息
二、排错过程
1、启动 unicorn 未监听端口
日志路径 : /var/log/gitlab/unicorn/unicorn_stderr.log
PG::ConnectionBad: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/var/opt/gitlab/postgresql/.s.PGSQL.5432"?
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:651:in `initialize'
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:651:in `new'
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:651:in `connect'
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:242:in `initialize'
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:44:in `new'
/opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activerecord-4.2.10/lib/active_record/connection_adapters/postgresql_adapter.rb:44:in `postgresql_connection'
··· ···
··· ···信息显示是因为连不上PG，所以启动 postgresql 后重启即可正常（嗯，是酱）。。
2、postgresql down
down: postgresql: 0s, normally up, want up; run: log: (pid 623) 15816094s
通过 PG 的日志路径 ： /var/log/gitlab/postgresql/current 可以查看到如下信息
2018-11-01_08:18:09.49669 FATAL: could not map anonymous shared memory: Cannot allocate memory
2018-11-01_08:18:09.49671 HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 4292984832 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.
2018-11-01_08:18:09.49671 LOG: database system is shut down也可以通过命令 `gitlab-ctl tail postgresql`，得到一样的信息，于是可以确定问题。。
This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 4292984832 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.
2018-11-01_07:52:06.63024 LOG: database system is shut down于是在配置文件中对 postgresql 的 shared_buffers 和 max_connections 两项进行了限制
[root@V2 ~]# cat /etc/gitlab/gitlab.rb |grep -v ^$ |grep -v ^#
external_url 'http://xxx.xxx.xxx.xxx.xxx:8090'
unicorn['worker_timeout'] = 60
unicorn['worker_processes'] = 3
unicorn['listen'] = 'xxx.xxx.xxx.xxx.xxx'
unicorn['port'] = 8870
postgresql['enable'] = true
postgresql['data_dir'] = "/var/opt/gitlab/postgresql/data"
postgresql['shared_buffers'] = "256MB" # ! **recommend value is 1/4 of total RAM, up to 14GB.**
postgresql['max_connections'] = 200
nginx['listen_addresses'] = ['*']
nginx['listen_port'] = 8090配置完成保存，之后更新配置，重启应用即可。
gitlab-ctl reconfigure # 更新配置
gitlab-ctl restart # 重启应用
gitlab-ctl status # 查看服务状态